Mass spectrometer



July 12, 1955 I. H. MCLAREN ET AL MASS SPECTROMETER 2 Sheets-Sheet 1 Filed Feb. 25, 1952 INVENTOR /AN/-/. MCLARE/V W/LL/AM C. W/LE Y July 12, 1955 1. H. MCLAREN ET AL 2,713,123

MASS SPECTROMETER Filed Feb. 25, 1952 2 Sheets-Sheet 2 4 g:- g EL. 1 4 4 4 4 4 6 4 4 Q 4 o 4 4 4 /5 4 4 is. g 5

INVENTOR BY MNH'M LAREN W/LL/AM C. WILEY MR W ATTORNEY United States Patent MASS SPEC'I'ROMETER Ian H. McLaren, Dearborn, and William C. Wiley, 13etroit, Mich., assignors to Bendix Aviation Corporation, Detroit, Mich, a corporation of Delaware Application February 25, 1952, Serial No. 273,198

7 Claims. (Cl. 250-413) This invention relates to mass spectrometers and more particularly to spectrometers for measuring the masses of different ions by subjecting them to rotational movements. The invention also relates to a method of rotating the ions through a predetermined number of revolutions so as to separate them on the basis of their mass.

Mass spectrometers are adapted in general to measure the masses of difierent ions and sometimes the relative abundance with which they occur. Some spectrometers perform such functions by producing a pulse of ions and subjecting the ions to a predetermined magnetic force. This force is applied on the ions in a direction substantially perpendicular to their initial movement, thereby causing the ions to travel through circular paths. The ions are collected preferably after rotating through a plurality of revolutions, rather than one revolution, since the increased travel path resulting from a plurality of revolutions causes the ions to be materially separated in space and the accuracy of the measurement to be increased. However, some of the ions of a particular mass tend to rotate through a difierent number of revolutions than the majority of ions of that mass as a result of their deviation from an optimum path. Such deviations of individual ions result from many factors, including the random motion imparted to the ions by their thermal energy. Because of these deviations, individual ions of a given mass may be collected at approximately the same time as other ions of a difierent mass so as to cloud the measurements which are obtained.

This invention provides a spectrometer which overcomes the above disadvantages by providing a guide path for the ions to follow during their rotational movement. The guide preferably has a helical configuration to direct the ions through a predetermined number of rotational orbits before the ions are detected in the detecting system. Since the ions rotate through only a predetermined number of revolutions before reaching a detector positioned adjacent to the outlet end of the guide, an increased clarity in the measurements of difierent ion masses is obtained over measurements in spectrometers now in use.

An object of this invention is to provide a mass spectrometer for determining the masses of different ions by subjecting a pulse of ions to a predetermined force and measuring the relative times at which the ions of different mass travel through a predetermined angular distance.

Another object is to provide a mass spectrometer of the above character for subjecting a pulse of ions to a rotational movement to separate them on the basis of their mass.

A further object is to provide a mass spectrometer of the above character for producing strong and accurate indications of the diiferent ion masses by subjecting the ions to a rotational movement through only a predetermined number of revolutions.

Still another object is to provide a mass spectrometer of the above character for guiding a plurality of ions through a predetermined circular path and for collecting the ions after their travel through this path.

A still further object is to provide a method of guiding a pulse of ions through a predetermined circular path so as to provide accurate measurements of the ion masses.

Other objects and advantages will be apparent from a detailed description of the invention and from the appended drawings and claims.

In the drawings:

Figure 1 is a perspective view illustrating the disposition of a mass spectrometer relative to a magnet for creating a magnetic field in the spectrometer;

Figure 2 is an enlarged perspective view of the interior of the spectrometer and shows in particular the relative disposition of an ion source for producing pulses of ions, a guide for directing the ions in a particular circular path through the spectrometer and a collector assembly for collecting the ions after they have travelled through the spectrometer;

Figure 3 is an enlarged perspective view illustrating in detail the ion source shown in Figure 2;

Figure 4 is an enlarged perspective view illustrating in detail the collector shown in Figure 2 and also showing in block form the measuring equipment associated with the spectrometer; and

Figure 5 is a diagrammatic view illustrating the particular path followed by the ions in the spectrometer as a result of the action of the guide shown in Figure 2.

In one embodiment of the invention, a permanent magnet, generally indicated at 10, is formed from a yoke 12 and oppositely disposed pole pieces 14 and 16 separated from each other by a suitable air gap. The magnet is adapted to supply a field of substantially uniform strength across the air gap. A mass spectrometer, generally indicated at 18, is located in the air gap between the pole pieces 14 and 16 and is defined by a cylindrical cover 20 (Figure 2) concentric with an inner core 22, the faces between the cylinders pressing against the pole pieces 14 and 16. The mass spectrometer has an ion source, generally indicated at 24; a collector, generally indicated at 26; and a guide, generally indicated at 28, cooperating with the ion source and the collector. The source 24 and the collector 26 are preferably tilted at slight angles to the magnetic field so that the ions which are produced will travel gradually downwardly as they describe their orbital paths.

The ion source 24 includes a filament 30 (Figure 3) made from a suitable material such as tungsten. The filament is connected to leads extending from terminals 32 in one of the faces between the cover 20 and the core 22 and is disposed at a negative potential with respect to ground. The filament 30 lies within an enclosure created by a grounded U-shaped bracket 34 connected as by screws to the cover 20 and faces a slot 36 in the portion of the bracket integrating the two legs of the U. A shield 38 having a slot corresponding to the slot 36 is suitably secured to the bracket 34 and is provided with a slot 40 in one of its walls transverse to the plane of the slot 36.

A plate 42 is secured to the shield 38 as by insulating brackets 44 and is provided with a slot 46 corresponding in shape and position to the slot 40. The slots 46 and 40 are adapted to pass ions which are produced from gas molecules introduced into the region within the shield 38 through a conduit 48. The conduit 48 fits at one end into the wall of the shield opposite the slot 40 and at the other end extends from a receptacle (not shown) adapted to hold molecules of the different gases constituting an unknown mixture.

The guide 28 is positioned adjacent the ion source 24 at one end at a level slightly above the slot 40 in the shield 38. The guide is formed from a sheet 50 of relatively thin and extended conductive material having a width corresponding substantially to the radial distance between the cover 20 and the core 22. The guide comprises a plurality of turns formed by wrapping the sheet 50 helically around the core 22 so that one turn lies above the other by a distance preferably greater than the height of the slots 40 and 46. The axial distance between turns also depends upon the angle at which the plate 42 and the shield 38 are cocked relative to the direction of the magnetic field.

At its other end, the guide 28 is positioned adjacent the collector 26. The collector is in turn preferably positioned on substantially the same axial line as the ion source 24 so that the ions will travel through a predetermined number of complete revolutions before they are collected. The collector 26 includes a shield 52 supported as by insulating brackets 54 to the spectrometer walls and a receptacle 56 positioned within the shield 52 and supported as by insulating brackets 58 to the shield. The shield 52 is grounded, and the receptacle 56 may have a relatively low positive potential applied to it to collect any secondary electrons emitted by it when the ions impinge on it.

The receptacle 56 is disposed within the shield 52 1n slightly recessed relationship to the shield and is slightly smaller than the shield. Both the shield 52 and thereceptacle 56 are parallelopipeds, with correspondlng sides of each parallelopiped being parallel to the corresponding side of the other parallelepiped. The side of each parallelopiped facing the flow of ions is open to receive the ions, and a grounded screen grid 60 extends across the open side of the shield 52. A time indicator 62, such as an oscilloscope, is connected to the receptacle 56 to provide an indication as to the relative times at which the ions of different mass reach the receptacle 56.

In operation, electrons are emitted from the filament 30 when the filament becomes heated by an electrical current passing through it. The electrons are attracted upwardly towards the slot 36 because of the positlve potential existing on the grounded bracket 34 relative to the negative potential on the filament. These electrons move through the slot 36 into the region within the shield 38, where the electrons strike gas molecules and ionize some of the molecules. The molecules of the dilferent gases are introduced into the region within the shield 38 from a receptacle (not shown) in predetermined proportions to the relative abundance of the gases in the re ceptacle.

Upon the formation of a relatively large number of ions, a negative pulse of voltage is applied to the plate 42 to accelerate the ions towards the plate. These ions pass in a bunch through the slot 46 in the plate and the slot 40 in the shield 38 into the magnetic field of the mass spectrometer. Because of the narrowness of the slots 40 and 46, the ions are emitted in a direction substantially perpendicular to the plane of the slots. The ions are also emitted in a direction largely perpendicular to the direction of the magnetic fiux extending between the pole pieces 14 and 16, since the plate 42 and the shield 38 are only slightly tilted with respect to the direction of the magnetic flux. This causes the magnetic field to exert a force upon the ions in a direction substantially perpendicular to both the magnetic field and the initial movement of the ions.

The force exerted on the ions by the magnetic field acts in a centripetal manner to produce a rotational movement of the ions. Since the shield 38 and the plate 42 are tilted at a slight angle, such as a few degrees, the ions tend to travel slightly downwardly in an axial direction during each revolution. The axial movement of the ions corresponds to the pitch of the guide 28 such that the ions follow the guide through the predetermined number of revolutions created by the guide and finally reach the collector 26. Each turn of the guide has a sufiiciently great height to tolerate without undue interference any random motion resulting from the internal and other energy in the ions.

As disclosed in co-pending application, Serial No. 83,258 filed March 31, 1949, by Dr. Samuel A. Goudsmit,

the time required for the ions of a given mass to rotate through a magnetic field is inversely proportional to the mass of the ions. This causes the ions of relatively light mass to travel through a predetermined number of revolutions before the ions of heavy mass and to impinge on the receptacle 56 before the ions of heavy mass. When the ions impinge on the receptacle 56, they may produce secondary emission of electrons from the receptacle because of their relatively high speed. The electrons produced by secondary emission are collected back at the receptacle because of the positive voltage provided on it relative to the voltage on the shield 52 and because of its confined nature. Thus, by measuring the charges appearing on the receptacle 56 and the times at which the charges appear, an indication can be obtained by the indicator 62 of the masses of the different gases in the unknown mixture and the relative abundance of each gas.

The mass spectrometer disclosed above has several important advantages. By subjecting the ions to a helical path, the ions travel through a relatively long distance to produce a distinct separation between the ions on the basis of their mass. As a result of the helical path through which the ions travel, the size of the mass spectrometer is minimized even though the travel path is long. The mass spectrometer also produces a finite separation between the ions of different mass as a result of the predetermined number of revolutions which all of the ions describe. This results from the action of the guide 28, which causes each of the ions to travel through a predetermined number of turns.

It should be realized that other ion sources than the source 24 may be used in the mass spectrometer disclosed above. For example, the source disclosed and claimed in co-pending application Serial No. 221,554, filed April 18, 1951, by Ian H. McLaren and William C. Wiley may also be used to optimum advantage. In addition, other ion collectors, such as a scintillation counter or electron multiplier, may also be used. It is even conceivable that detectors may be used which are actuated by the ions without collecting the ions, after the ions have travelled through the predetermined distance. In addition, the collector 26 may be positioned relative to the source 24 so that the ions travel through an angular distance which is greater than 360 but not necessarily an integral multiple of 360". A mass spectrometer for collecting the ions after their travel through an orbit diiferent from an integral number of complete revolutions is disclosed and claimed in co-pending application Serial No. 235,744, filed July 9, 1951 by William C. Wiley.

Nor is the spectrometer constituting this invention limited to the particular construction of the guide 28 disclosed above, as long as the guide directs the ions through an optimum path constituting a predetermined number of revolutions. Furthermore, although the ion source 24 and the collector 26 are preferably tilted at a slight angle to the magnetic field, the angle of tilt may be increased provided that the pitch of the guide 28 and the axial length of the spectrometer are correspondingly increased. For example, the tilt may be increased when the mass spectrometer is used as a leak detector to find leaks in equipment by passing ions of a given mass through the equipment. The thermal energy of the ions may also sometimes be employed to impart a helical motion to the ions and in such a situation the source 24 and collector 26 would not have to be tilted at all.

Although this invention has been disclosed and illustrated with reference to particular applications, the principles involved are susceptible of numerous other applications which will be apparent to persons skilled in the art. The invention is, therefore, to be limited only as indicated by the scope of the appended claims.

What is claimed is:

l. A mass spectrometer, including, means for emitting tion, means for producing a magnetic field in a direction substantially perpendicular to the first direction to subject the ions to a rotational movement, a guide for directing the ions through a predetermined rotational path, a collector disposed substantially perpendicular to the direction of ion flow and substantially parallel to the direction of the magnetic field to receive the ions after their rotational movement, and an indicator for determining the relative times at which the ions of difierent mass reach the collector.

2. A mass spectrometer, including, means for emitting ions of different mass in substantially a predetermined direction, means for subjecting the ions to a magnetic field providing lines of flux in a direction substantially perpendicular to the direction of ion emission so as to produce a rotation of the ions, means for guiding the rotational movement of the ions through a helical path, means for collecting the ions upon their rotation through the helical path, the collecting means being disposed substantially perpendicular to the direction of ion movement and substantially parallel to the direction of the magnetic field, and means for measuring the times of flight of the ions to determine their mass.

3. A mass spectrometer, including a cover having an axis in a first direction, means for introducing ions to the region within the cover, means for producing a magnetic field in a direction substantially parallel to the axis of the cover so as to subject the ions to a rotational movement, a guide for directing the ions through the region within the cover in a helical path having an axis corresponding to the axis of the cover, a collector disposed at the end of the guide in a direction corresponding to the axis of the cover to receive the ions after their movement through the helical path, and means for indicating the relative times at which the ions of different mass reach the collector.

4. A mass spectrometer, including a cover having an axis in a first direction, means for providing magnetic flux lines through the region within the cover in substantially the same direction as the axis of the cover, means for introducing a plurality of ions having difierent masses into the region within the cover in a direction having a component perpendicular to the axis of the cover, means extending at one end from the introducing means and forming with the cover a helical guide for directing the ions in a predetermined path through a plurality of revolutions, the helical guide means having an axis corresponding to the axis of the cover, means facing the guide means to collect the ions passing through the guide means, the collecting means being disposed in a direction having an axis corresponding to the axis of the cover, and means for determining the relative times at which the ions of different mass reach the collecting means.

5. A mass spectrometer, including, an annular cover, means for providing magnetic flux lines through the region within the cover in substantially the same direction as the axis of the cover, an ion source positioned in the cover along part of its axial length and operative to introduce a pulse of ions having difierent masses into the region within the cover in a direction substantially perpendicular to the axis of the cover, means extending at one end from the introducing means and forming with the cover a helical guide for directing the ions through a predetermined plurality of revolutionsfthe helical guide having an axis corresponding substantially to the axis of the cover, a collector positioned in the region within the cover along substantially the same axial line as the cover and adjacent the other end of the guide means to receive the ions passing through the guide means, and means for indicating the relative times at which the ions of different mass reach the collector.

6. A mass spectrometer, including, means for producing a magnetic field, means tilted slightly in the direction of the magnetic field and adapted to emit ions into the magnetic field for a rotary movement through the magnetic field, means for guiding the ions through a helical path having a pitch corresponding substantially to the pitch of the emitting means, means tilted slightly in the direction of the magnetic field at an angle corresponding to the angle of the emitting means and adapted to collect the ions after their movement through the helical path, and means for indicating the relative times at which the ions of different mass reach the collecting means.

7. A mass spectrometer, including, means for producing a magnetic field, an emitter tilted at a relatively small angle in the direction of the magnetic field and adapted to emit ions into the magnetic field for a rotary movement through the magnetic field, a guide for directing the ions through a predetermined number of revolutions in a helical path having a pitch corresponding substantially to the pitch of the emitter, a collector tilted in the direction of the magnetic field at an angle corresponding to that of the emitter and adapted to collect the ions after their movement through the predetermined number of revolutions, and an indicator for indicating the relative times at which the ions of different mass reach the collector.

References Cited in the file of this patent UNITED STATES PATENTS 2,378,962 Washburn June 26, 1945 2,576,601 Hays Nov. 27, 1951 2,582,216 Koppius Jan, 15, 1952 2,698,905 Goudsmit Jan. 4, 1955 

